1. Summary of the Invention
The present invention relates to a gene, CBF1, encoding a protein, designated as CBF1, which binds to a region regulating expression of genes which are activated during acclimation to cold temperature and drought. The gene is expressed in microorganisms and can be used to produce recombinant plants.
2. Description of Related Art
Environmental factors serve as cues to trigger a number of specific changes in plant growth and development. One such factor is low temperature. Prominent examples of cold-regulated processes include cold acclimation, the increase in freezing tolerance that occurs in response to low non-freezing temperatures (Guy, C. L., Annu. Rev. Plant Physiol. Plant Mol. Biol. 41:187-223 (1990)); vernalization, the shortening of time to flowering induced by low temperature (Lang, A., in Encyclopedia of Plant Physiology, Vol. 15-1, ed. Ruhland, W. (Springer, Berlin), pp. 1489-1536 (1965)); and stratification, the breaking of seed dormancy by low temperature (Berry, J. A. and J. K. Raison, in Encyclopedia of Plant Physiology, Vol. 12A, eds. Lange, O. L., Nobel, P. S., Osmond, C. B. and Ziegler, H. (Springer, Berlin), pp. 277-338 (1981)). Due to the fundamental nature and agronomic importance of these processes, there is interest in understanding how plants sense and respond to low temperature. One approach being taken is to determine the signal transduction pathways and regulatory mechanisms involved in cold-regulated gene expression.
Strong evidence exists for calcium having a role in low temperature signal transduction and regulation of at least some COR (cold-regulated) genes. Dhindsa and colleagues (Monroy, A. F., et al, Plant Physiol. 102:1227-1235 (1993); Monroy, A. F., and R. S., The Plant Cell, 7:321-331 (1995)) have shown that, in alfalfa, calcium chelators and calcium channel blockers prevent low temperature induction of COR genes and that calcium ionophores and calcium channel agonists induce expression of COR genes at normal growth temperatures. Similarly, Knight et al (The Plant Cell 8:489-503 (1996)) have shown that cold-induced expression of the Arabidopsis thaliana COR gene KIN1 is inhibited by calcium chelators and calcium channel blockers. These results suggest that low temperature triggers an influx of extracellular calcium that activates a signal transduction pathway that induces the expression of COR genes. Consistent with this notion is the finding that low temperature evokes transient increases in cytosolic calcium levels in plants (Knight, M. R. et al, Nature 352:524-526 (1991); Knight, H., et al., The Plant Cell 8:489-503 (1996)). In addition, low temperatures have been shown to stimulate the activity of mechanosensitive calcium-selective cation channels in plants (Ding, J. P. and B. G. Pickard, Plant J. 3:713-720 (1993)).
Recent efforts have led to the identification of a cis-acting cold-regulatory element in plants, the C-repeat/DRE (Yamaguchi-Shinozaki, et al., The Plant Cell 6:251-264 (1994); Baker, S. S., et al., Plant. Mol. Biol. 24:701-713 (1994); Jiang, C., et al., Plant Mol. Biol. 30:679-684 (1996)). The element, which has a 5 base pair core sequence for CCGAC, is present once to multiple times in all plant cold-regulated promoters that have been described to date; these include the promoters of the COR15a (Baker, S. S. , et al, Plant. Mol. Biol. 24:701-713 (1994)), COR78/RD29A (Horvath, D. P., et al, Plant Physiol. 103:1047-1053 (1993); Yamaguchi-Shinozaki, K., et al., The Plant Cell 6:251-264 (1994)), COR6.6 (Wang, H., et al., Plant Mol. biol. 28:605-617 (1995)) and KIN1 (Wang, H., et al, Plant Mol. Biol. 28:605-617 (1995)) genes of Arabidopsis and the BN115 gene of Brassica napus (White, T. C., et al, Plant Physiol. 106:917-928 (1994)). Deletion analysis of the Arabidopsis COR15a gene suggested that the CCGAC sequence, designated the C-repeat, might be part of a cis-acting cold-regulatory element (Baker, S. S., et al., Plant Mol. Biol. 24:701-713 (1994)). That this was the case was first demonstrated by Yamaguchi-Shinozaki and Shinozaki (Yamaguchi-Shinozaki, K., et al., The Plant Cell 6:251-264 (1994)) who showed that two of the C-repeat sequences present in the promoter of COR78/RD29A induced cold-regulated gene expression when fused to a reporter gene. It was also found that these two elements stimulate transcription in response to dehydration and high salinity and thus, was designated the DRE (dehydration, low temperature and high salt regulatory element). Interestingly, two other C-repeats present in the promoter of COR78/RD29A did not impart regulation in response to dehydration and high salt; whether they impart cold-regulated gene expression is not known. Recent studies by Jiang et al (Jiang, C., et al., Plant Mol. Biol. 30:679-684. (1996)) indicate that the C-repeats (referred to as low temperature response elements) present in the promoter of the B. napus BN115 gene also impart cold-regulated gene expression, but do not activate gene expression-in response to dehydration.
U.S. Pat. Nos. 5,296,462 and 5,356,816 to Thomashow describe the genes encoding the proteins involved in cold regulation in Arabidopsis thaliana. In particular the DNA encoding the COR15 proteins is described. These proteins are significant in promoting cold tolerance in plants; however, the mode of activation is not described.
There is a need for a protein which regulates the expression of genes encoding cold and drought tolerance genes.